There has long been a need for new receivers designed to monitor a selected frequency band, simultaneously perform spectral analysis of multiple signals appearing at unpredictable frequencies in the band and simultaneously coherently extract the multiple signals. A receiver which performs the first function, spectral analysis, is the compressive or microscan receiver. Such receivers are best described in the publications:
1. H. M. Gerard, O. W. Otto, "Chirp Transform Opens New Processing Possibilities", Microwave System News, pp. 85-92, October, 1977.
2. J. B. Harrington, R. B. Nelson, "Compressive Intercept Receiver Uses SAW Devices," Microwave Journal, pp. 57-62, 1974.
Suffice it to say, conventional compressive receivers form the analog Fourier transform of their input, with both amplitude and phase preserved prior to envelope detection. This transform is produced once per scan period (T.sub.S) with a frequency resolution approximately 1/T Hz, where T is the length of time a pure tone at the receiver input is visible to the compression filter. For conventional receivers T.sub.S &gt;T.
Typically, the fraction of sweep time (T.sub.S) that a signal is visible to its compression filter termed probability of intercept (POI) or duty cycle for a conventional compression receiver is 0.5 or 50%. Only in limiting cases where the input bandwidth approaches 0 Hz or the scan rate R becomes infinite does the POI approach 1 or 100%. The end result has been that conventional receivers necessarily have scan rates which are too slow relative to the Nyquist criteria and channelization of signals is therefore prohibited.
A somewhat different approach has been taken by certain researchers employing conventional compression receivers in which the compression filter bandwidth and dispersion are twice that of the swept oscillator.
Such a configuration approaches minimum Nyquist sampling since T.perspectiveto.T.sub.s. However, each output pulse is located at a different intermediate frequency, corresponding to its initial location within the input bandwidth. Thus, side-lobe supression in this system cannot be achieved by a passive IF filter, and the noise bandwidth is twice that required for the same resolution in the previous configuration. Additionally, simultaneous channelization of signals of interest (SOI) is not possible due to the random IF's.
As a final comment on the prior art, we note that it is not unusual to find a signal of interest in close frequency proximity to an unwanted strong emmiter. Under these conditions, it is very likely that energy from this strong signal will be significant within the channelization filter centered on the signal of interest. Thus, in practical situations, it is important to have the receiver sampling rate significantly above Nyquist (say twice) to avoid folding the strong emmiter directly onto the signals of interest. In systems of this type, replicating the receiver to provide parallel, time-sequential channels is required. This is a costly approach in terms of both hardware and the degree to which the multiple receivers must be matched.